Worrying about making ends meet, it seems, can occupy enough of the brain‘s finite thinking power that it makes it difficult to think clearly.

According to the latest research published in Science, just thinking about shaky finances can drop IQ by the equivalent of 13 points. That may help to explain why poverty can become a vicious cycle, with lower income people tending to make seemingly irrational and risky decisions, particularly when it comes to money.

To determine how budgetary concerns affect thinking, the researchers examined the effects of financial strain among both a group of shoppers at a New Jersey mall and impoverished sugarcane farmers in rural India. The mall visitors had household incomes ranging from $20,000 to $150,000, with a median income of $70,000. The farmers were relatively flush with cash at harvest— but desperately poor for most of the rest of the year.

The shoppers considered a range of financial difficulties, from having to take small pay cuts to larger ones, or to suddenly being faced with minor or more expensive car repairs. They were asked about how they would cope with such problems— by borrowing, cutting spending or skipping the car repair and hoping for the best. Then they took tests to measure IQ and their cognitive skills.

When confronted with relatively minor financial problems, the lower income people performed equally well on the tests as higher income folks. But when faced with more serious financial concerns, the lower income individuals did much worse than their wealthier counterparts. In fact, in one version of the experiment where participants were paid for each correct answer, the rich earned 18% more than those who weren’t as well off.

The findings among the Indian sugarcane farmers were almost as strong. The researchers tested 464 farmers before and after harvest, when their finances were drastically different. When the farmers had cash after harvest, they performed well on the cognitive tests. But before harvest, when money was scarcer, they did much worse— showing a decline similar to the loss of 10 IQ points.

With half the American population living from paycheck to paycheck, the study’s lead author Eldar Shafir, professor of psychology and public affairs at Princeton University, says the findings are relevant to understanding how financial circumstances influence intellectual ability.

“There’s always been this perception that the poor function less well,” says Shafir, “But it’s not the person, it’s the situation they’re in and anyone could find themselves there.” Previous studies have found the poor to be generally less productive, less attuned as parents, and to have lower IQs— findings that are apparently linked to the stress of poverty. But those studies also left the impression that these factors might be causes of poverty, while the latest research suggests that they may be the result.

“These authors came up with very clever, elegant research designs that give us strong evidence about one cause of the sometimes counterproductive behaviors of the poor,” says Martha Farah, director of the Center for Neuroscience and Society at the University of Pennsylvania, who was not associated with the study.

And while it’s tempting to think stress explained these results, that wasn’t the case. Even after controlling for stress hormone levels, researchers found that the poor did worse.

“When you don’t have enough [money], it occupies your mind and takes away bandwidth that you could use for other things,” says Shafir, in explaining the findings. And numerous studies confirm that when mental load increases, decision-making quality goes down. That’s why people tend to make worse choices at the end of the day— or after making multiple decisions, even if some were trivial.

Can you overcome the problem by not allowing yourself to be overwhelmed, or by convincing yourself that you have more mental thinking power than you actually do? While some studies found that such mental gymnastics could boost cognitive bandwidth, they may only work up to a point. Positive thinking alone, it seems, isn’t enough to tackle a heavy mental load.

But actively making “space” in your brain to address challenges as they occur might be one way to avoid compromising your thinking skills. Creating routines, or default choices — such as what you eat for breakfast, or what you wear for certain events — can leave your brain ready to take on unexpected problems without getting overwhelmed. And scheduling events that require decision-making earlier in the day, before experiences and worries occupy your attention, could also help. Recognizing that worry can be distracting is also essential, so avoiding important choices when you’re focused on something stressful also makes sense, says Shafir. Making smart choices is about more than just being smart — it also involves being in the right state of mind to let your cognitive powers do their thing.